Adopted Roads

[Cambsguy]

Hello. I am hoping for some advice about car parking on a road. We are a small culdesac of houses with out house at the end on what is our private road which we pay to upkeep. We had a freind visit recently who had to park his car on the main road which as far as we know is not a private road but an adopted one. When my friend returned to his car he found a piece of paper on his windscreen with a hand written message asking to please not park here again. He was not parked outside their house or blocking any access gates. He parked on the other side of the road next to a house that has a very high wall. The road has no official markings as such. Are they allowed to stop people parking? It seems they have an attitude that the road outside their garden is theirs. I don’t know where to find out who owns it. We think they are trying it in because we are new here. Any help appreciated.